He did not say "exactly once." When he said ONCE, I knew he meant "at least once" which is true even if he had sex 1000 times. EX42 studied his logic.
There is another possibility to consider. Clinton bragged more than once to friends that Gennifer could suck a tennis ball through a garden hose. Now, I'm not sure what that means, but it could just possibly faintly hint that he had a lot of "not-sex" of the sort made famous by Monica Blewinski and Nina Burleigh. This is a good kind of sex to have (or rather, not have) because all the Clintonites insist that it is not sex (at least if you are talking to a US Special Attorney), so if you have sex-that-isn't-sex, you can say you didn't have sex. Yet in higher liberal circles if you have oral sex-that-isn't sex, you are REQUIRED to lie about it (especially to US Special Prosecutors) because, see, everybody lies about sex, even when it's not sex.
I trust this makes perfect sense and answers all your questions?